1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to a mechanism that resolves a density deviation in a toner image formed on a latent image carrier.
2. Description of the Related Art
As is generally known, in electrophotographic image forming apparatuses, electrostatic latent images formed on photoconductors, which are latent image carriers, are converted to visible images with toners, and toner images are transferred and fixed onto recording media (hereinafter, referred to as recoding materials for convenience) such as recording paper to obtain copied outputs.
Image forming apparatuses not only include those having monochrome image forming configurations but also those having configurations for forming images of plural colors, like full color images. In such a color image forming apparatus, images of different colors formed in plural image forming units are sequentially transferred onto an intermediate transfer body such as a belt (primary transfer), and thereafter superimposed images are collectively transferred onto a recording material (secondary transfer).
For a developing device used in a visible image process, a two-component developer including a toner and magnetic particles that function as a carrier, or a single-component developer not including a carrier is used.
When the single-component developer is used, the toner is charged when the toner passes a layer thickness regulation blade. However, since it is difficult to saturate a toner attachment amount on a developing roller by one-time feeding from a feeding roller to a developing roller, a so-called development history (ghost) occurs where an image density upon continuous printing of a solid image of a large area becomes different from a density of a solid image directly after a large non-image region.
This development history (ghost) corresponds to a phenomenon in which a so-called unnecessary after-image, for example, an after-image of a character within a halftone image, that is, an image of a different density, appears. It has been disclosed that this is caused by the difference in triboelectricities (charge quantities per unit area) of toners on the developing toner, that is, the difference in triboelectricities between the toner facing an image (a developing part) and a toner facing a non-image part (a non-developing part) (for example, see Japanese Patent Application Laid-open No. 09-106175). That is, the toner facing the non-developing part is not transferred toward an electrostatic latent image for development and thus is rubbed against the layer thickness regulation blade more frequently than the toner in the part transferred toward the electrostatic latent image. As a result, the triboelectricity of the toner in the non-developing part increases more than the toner in the developing part, leading to the density unevenness.
Thus, when the single-component developer is used, a velocity of a feeding roller may be increased in an attempt to increase a velocity of feeding the toner from the feeding roller to the developing roller, but if this is done, heat is generated in a contact portion between the developing roller and the feeding roller, and thus the toner on the regulation blade may be fixed to the developing roller, or toner deterioration may be accelerated.
Further, even if the abutting pressure between the feeding roller and the developing roller is increased in order to increase a toner feed quantity or the abutting pressure between the regulation blade and the developing roller is increased in order to obtain saturation with a low attachment amount, the fixing or toner deterioration may be similarly accelerated.
Meanwhile, increasing the number of feeding rollers in order to increase the feed quantity is disadvantageous in terms of volume (downsizing) of a developing machine. Particularly, in recent single-component developing machines, line speeds of the developing rollers have increased more and more due to a demand for higher printing speeds, and it has been difficult to saturate feeding of a toner from a feeding roller to a developing roller at once due to heat generation. Because of these problems, the single-component developing system is hardly employed in printing devices requiring high image qualities.
When the two-component developer is used, because stirring and mixing of the carrier and the toner are able to be sufficiently performed, the above problems occurring with respect to the single-component developer do not occur as much, but the above developing history problem still occurs.
Particularly, when the developing part reaches a developing position directly after the non-developing part, the density of a front end of an image part in an image transferred onto a recording material becomes high as illustrated in FIG. 22.
The inventors of the present application have studied the phenomenon of the front end of the image part directly after the non-image part becoming dense and found out that when a developing sleeve used as a developer carrier is positioned directly after the non-image part, the toner is attached to a surface thereof.
When performing reversal image forming of removing a charge on the photoconductor corresponding to the image part and attaching the toner having the same polarity as the non-image part of the photoconductor by a developing bias, as the non-image part passes the developing sleeve, the toner receives a force moving toward the developing sleeve due to an electric field formed by the charge remaining in the non-image part on the photoconductor. Accordingly, the toner attachment amount on the surface of the developing sleeve increases.
In the image part directly after the non-image part on the photoconductor passes the developing sleeve at which the toner attachment amount has increased, more toner than a toner attachment amount of a predetermined density is attached, and density abnormality at the front end of the image part directly after the non-image part occurs. More particularly, the density becomes higher than the density in the image part other than the front end of the image part.
The deviation in the density at the front end of the image part directly after the non-image part becoming abnormal, that is, the development history that becomes the so-called ghost, is considered to be caused when a development history of a previous round of the developing sleeve remains on the developing sleeve in the developing process, and a development history of the first round is generated as an after-image during development of a second round.
As a method of resolving the density deviation, a method of reducing the density deviation by providing an electrode plate facing a developer carrier and applying a vibration electric field to the electrode plate to rearrange a toner on the carrier and destroy an after-image has been proposed (for example, see Japanese Patent Application Laid-open No. 09-106175).
Further, other methods are available, including a method of regulating a particle diameter distribution and a circularity of a carrier and reducing density unevenness by using a method of carrying a developer similar to the method of carrying the two-component developer via the carrier and using a method similar to the visible image process for using the single-component developer in which the toner flies to the latent image at the portion facing the image part (for example, see Japanese Patent Application Laid-open No. 2007-264336), a method of removing a condition in which a development history is generated by peeling off a remaining toner from a developer carrier after development (for example, see Japanese Patent Application Laid-open No. 2007-86448), and a method of eliminating density deviation by adjusting a stirring time in a developing unit before development according to the number of accumulated printed sheets and eliminating a charge amount difference on a developer carrier with respect to a non-image part and an image part (for example, see Japanese Patent Application Laid-open No. 2006-220749).
Further, other methods have been proposed, including a method of preventing a density at a front end of an image part directly after a non-image part from increasing, a method of controlling a voltage applied to a layer thickness regulation member for a single-component developer (for example, Japanese Patent Application Laid-open No. 2005-189767), and a method of setting a surface material of a developer carrier or controlling a potential in a non-image region so that a charge of a toner does not change (for example, see Japanese Patent Application Laid-open No. 2003-84504).
All of the methods disclosed in the above described patent documents relate to single-component developers and are not targeted on the two-component developers.
The friction charging used for the two-component developers is different from that for the single-component developers, and thus the above-described methods are not directly applicable.
Meanwhile, when the two-component developers are used, similarly to the single-component developers, the density deviation in an image occurs at the front end of the image part directly after the non-image part as described above.
Particularly, the inventors have confirmed that the density deviation occurs even if the developer is peeled off from the developer carrier by a magnetic force in using the two-component developers. Therefore, it is difficult to completely remove the development history upon use of the two-component developers even if the above-described methods are used.